Photomultiplier tubes are very sensitive light sensors, especially for visible radiation. FIG. 1 is a schematic showing the electrical circuit used to bias the photomultiplier and form the output voltage signal 24. Light is incident on the photocathode 12 having a cathode 22 and an anode 20. The resulting photoelectrons are accelerated to a series of dynodes 14, 16, 18 to generate secondary electrons and through this electron multiplication amplify the signal. Gains of 10.sup.8 can be achieved with only minor degradation of the linearity and speed of vacuum photodiodes. The spectral response is governed by the emission properties of the photocathode.
There are various types of photomultipliers with different physical arrangements to optimize for specific applications. The high voltage supply ranges from 300 to 3000 V, and the electron multiplication gain is normally adjusted by varying the supply voltage. The linearity of a photomultilpier is very good, typically 3% over three decades of light level. Saturation is normally encountered at high anode currents caused by space charge effects at the last dynode where most of the current is generated. The decoupling capacitors, C.sub.i, on the last few dynodes are used for high frequency response to prevent saturation from the dynode resistors.
Scintillation probes all utilize a photomultiplier tube of some type. The photomultiplier tube functions properly as long as the probe is light tight. Gamma scintillation probes provide the light tight integrity with metal canning around the probe. This is acceptable since gamma radiation can penetrate the canning of the probe and get counted. Also a gamma background is present at all times so a positive indication is normally present on the meter. When a light leak occurs in the canning the photomultiplier tube becomes saturated and the meter response goes to zero.
Alpha scintillation probes, however, require a thin mylar covering to allow the alpha particles to enter the probe. An alpha particle is a helium nuclei. A thin mylar covering can effectively block visible light; however, the thin mylar is easily damaged during normal surveys. Alpha background normally does not exist and the meter indicates zero. This is a problem since the lack of a meter response may also be due to the photomultiplier tube being saturated from a light leak. Instrument manufacturers have attempted to solve this problem by using a balanced circuit to detect small changes in the output current of the photomultiplier tube. The balanced circuits have proven to be very unstable and have caused difficulty in calibration of the instrument.
Low level radioactive alpha sources attached to the instrument have been the only reliable method to check for proper operation of the meter. The disadvantages of this method are: 1) the handling and maintenance of the sources with the instrument, 2) the probe must be removed from the survey location to check for a meter response with the source, and 3) the low level alpha source emits few alpha particles in a random pattern so the probe must be held on the source until a positive indication is made.
A need exists for a saturation indicator for an alpha contamination probe which overcomes the disadvantages found in the prior art.